1. Field of the Invention
The present invention relates to single-hop multichannel communication networks in general, and more specifically, to a power-efficient, non-bus-oriented single-hop interconnect network architecture for optical signals.
2. Discussion of the Related Art
The evolution of modern data communications networks has steadily increased the demand for networks offering high data transmission speeds and high levels of data parallelism or channel concurrency. Data transmission rates are limited by the physical technology composing the network interconnection linkages. Channel concurrency is limited by the requirement that multiple transmissions remain distinguishable within the network when routed to the appropriate destinations. With standard bus-oriented network architectures, the number of concurrent transmissions is less than or equal to the number of buses.
A bus is a device used to completely interconnect a set of transmitters to a set of receivers. A network is said to be bus-oriented if all connections are made by buses and each transmitter and receiver is on only one bus. Optimal concurrency requires a protocol for transmitting data packets through a network without packet-to-packet interference. Ideally, a useful concurrency protocol should slow data transmission rates as little as possible, although all transmissions may be completely scheduled in advance or may occur according to an appropriate conflict resolution rule.
The prior art is replete with bus-oriented single-hop interconnection (SHI) techniques for improving channel concurrency within a communication network. Such techniques are not limited to any particular physical communication technology. Recent improvements in fiber optic transmission technology and the invention of the optical star coupler have given rise to explosive growth of optical network applications. Optical data transmission technology is favored because of the very high data transmission rates possible at optical frequencies. Unfortunately, optical bandwidth does little by itself to improve channel concurrency in switched networks. Without circuit components capable of optical switching speeds, concurrency limitations will continue to be an obtrusive handicap for optical data transmission networks.
Not surprisingly, practitioners in the art have suggested improved non-switched interconnection techniques for overcoming the limitations of bus-oriented optical interconnection. Some proposals were intended to overcome inherent limitations of optical interconnection devices, such as star couplers. For instance, in U.S. Pat. No. 4,543,666. Hans-Hermann Witte, et al. disclose a method for interconnecting N subscriber transmitters to N subscriber receivers using a plurality of optical star couplers. Witte, et al, use active optical repeaters to overcome the effects of line attenuation. Their invention exploits the directionality of star couplers to provide an echo-free bus-oriented interconnection architecture that improves channel capacity by simplifying the necessary bus protocol. However, as with optical switching devices, the use of active optical repeaters increases the costs and reduces the reliability of their bus-oriented network. Recent publicized telephone system failures in the U.S. highlight the effects of these problems.
In U.S. Pat. No. 4,914,648, Anthony Acampora, et al, disclose a bus-like Multiple Hop Interconnection (MHI) multichannel network that avoids the need for agile optical switching devices. Their channels are all bus-like because any two transmitters are either connected to exactly the same receivers or to disjoint sets of receivers. Acampora, et al, use a perfect shuffle network to simplify the necessary protocols under uniform traffic but they also pay the price of slower data transmission because of the multiple hops (repeated active packet transmissions) required by their invention.
For a discussion of bus-oriented and non-bus-oriented SHI networks, see Matthew T. Busche, et al, "On Optical Interconnection of Stations Having Multiple Transmitters and Receivers," 1990 International Symposium on Information Theory and its Applications (ISITA '90), Hawaii, U.S.A., Nov. 27-30, 1990, session 63-3, pp. 967-970. See also, Y. Birk, et al, "Bus-Oriented Interconnection Topologies for Single-Hop Communications among Multi-Transceiver Stations," IEEE Infocom'88, pp. 558-567, IEEE Computer Society Press, 1988. For an early discussion of non-bus-oriented networks, see Y. Birk, "Concurrent Communication Among Multi-Transceiver Stations Over Shared Media", PhD Dissertation, Stanford University, Dec. 1986.
As used in the art and discussed by Birk, uniform traffic means substantially equal data traffic across different "types" of source station transmitters. Scheduled traffic implies a round-robin transmission schedule and not a random or "on-demand" schedule. Strictly speaking, this means that there is the same amount of traffic between any source/destination pair. However, so long as the correct "types" of source stations transmit in each time slot and the destination are as scheduled, the full channel concurrency will be attained. Thus, if one source station of a given type has above average traffic for a given destination and another source station of the same type has less traffic for the same destination, the schedule can be modified to allocate additional slots to the former at the expense of the latter. A similar argument applies if source stations have a single transmitter and destination stations have multiple receivers. Moreover, even if the traffic is not exactly uniform, the resulting degradation in concurrency is gradual because only some of the time slots are underutilized. To operate an interconnection network with a given schedule, it is necessary to synchronize the stations so they know when to begin transmitting. This can be done using a central "clock" whose signal is distributed to all source station or by any other suitable means known in the art.
Multichannel capacity is defined as the product of data rate and concurrency, as is known in the art. Busche, et al, observe that m source stations having p transmitters per station can be interconnected with a destination stations having one or more receivers per station using either bus-oriented or non-bus-oriented SHI's. However, because maximum possible transmission rate is believed to be inversely proportional to the power loss along the path, they suggest that the optical power-split losses in passive non-bus-oriented SHI multichannel networks using multiple optical star couplers will limit the maximum possible multichannel capacity to less than the capacity already available in bus-oriented SHI networks.
Birk discusses non-bus-oriented SHI's that are organized so that each transmitter of a source station (SS) is directly connected to some set of destination stations (DS's), where the sets of DS's for different SS's can be chosen independently of one another. This concept differs from bus-oriented SHI's, which require the sets of DS's connected to any two transmitters, one from each of two distinct SS's, to be either identical or disjoint. Succinctly, a bus-oriented SHI is limited to a concurrency of p (number of transmitters per SS) but has an optimal power spreading loss factor of n/p.
For SS's having p=2 transmitters, a passive non-bus-oriented SHI of m SS's to n DS's with direct connections from each SS to every DS is limited in size to about n=20 or so because the power transmitted by each SS is divided by n.sup.2 /2 at every DS. This division occurs because of the two star coupler stages needed to make such a connection. A first (1 by n/2) star coupler splits the signal transmitted from a single SS transmitter to n/2 branches and a second (n by 1) star coupler joins all SS outputs destined for a single DS input. As is known in the optical art, the star coupler introduces an optical power loss equal to the maximum of the number of outputs or inputs. Thus, in this example, both first and second coupler stages reduce optical signal power by n. The total network power-split loss is then n*n/2 or n.sup.2 /2 .
Increasing the number of optical transmitters to p at each SS will decrease the number of receivers that must sense a single transmitter, thereby increasing the power available at the DS receiver and reducing the effective power loss factor to n.sup.2 /p. Adding active devices can also increase available network connectivity, but only by increasing cost, complexity and reliability problems. Succinctly, such a non-bus-oriented SHI offers an improved concurrency of k=( log.sub.p n choose (p-1)=( log.sub.p n )!/(p-1)!/( log.sub.p n -p+1)! but suffers with an optimal power spreading loss factor of n.sup.2 /p.
There is a strongly felt need in the optical network art for such a high-concurrency passive SHI technique for interconnecting numbers of source stations and destination stations well above the existing practical limit of n=20. Increasing n is desired because it leads to increased non-bus-oriented multichannel capacity resulting from improved concurrency k. Of course, the theoretical upper limit on passive network size is governed the same linear power spreading loss factor n/p known for bus-oriented networks because each SS within a passive SHI network must be connected to every DS by a single-hop link. However, reducing the power spreading factor from n.sup.2 /p to n/p would vastly improve passive network channel capacity at higher values of maximum concurrency k. Thus, a need is felt for an interconnection wiring technique that will reduce power spreading losses in passive non-bus-oriented SHI's from a factor of n.sup.2 /p to a factor closer to the theoretically optimum value of n/p. The associated problems and unresolved deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.